1. Field of the Invention
The present invention relates to a nonvolatile memory device and a method of manufacturing the same, and more particularly, to a nonvolatile memory device having an increased trap site density and a method of manufacturing the same.
2. Description of the Related Art
A unit memory cell of semiconductor memory devices, such as DRAM, may include one transistor and one capacitor. Therefore, to increase the integration density of a semiconductor memory device, the volume of the transistor and/or the capacitor must be reduced.
In the case of early memory devices when the integration density was not a big issue, photography and etching processes could be performed with sufficient process margins. Therefore, the integration density of semiconductor memory devices could be increased by reducing the volume of each element of the semiconductor memory device.
However, new methods are required as the demand for semiconductor memory devices having high integration density increases.
The integration density of a semiconductor memory device is closely related to a design rule. Therefore, to increase the integration density, a strict design rule must be applied. In this case, the process margins of photography and etching processes can be significantly reduced, meaning that these processes must be performed more precisely.
When the process margins of the photography and etching processes are reduced, the yield can also be reduced. Therefore, a method to increase the integration density of semiconductor memory devices without reducing the yield is needed.
To meet these requirements, many semiconductor memory devices having different structures from the conventional semiconductor memory devices have been introduced. These include a data storage medium that can store charge on the upper side of the channel of a transistor and has a different data storing function from a conventional capacitor.
A SONOS memory device is another newly introduced semiconductor memory device. FIG. 1 is a cross-sectional view illustrating a conventional memory device.
Referring to FIG. 1, a source region 12 and a drain region 14 to which an n type conductive dopant is implanted on a p type semiconductor substrate 10 (hereinafter, semiconductor substrate) are formed. A channel region 16 is formed between the source 12 and the drain 14. Also, a gate stack 18 is formed on the channel region 16 of the semiconductor substrate 10. The gate stack 18 is composed of a tunneling oxide film 18a, a nitride film Si3N4 18b, a blocking oxide film 18c, and a gate electrode 18d. Here, the nitride film 18b has trap sites. Therefore, when a voltage is applied to the gate electrode 18d, electrons pass through the tunneling oxide film 18a and are trapped in the trap site of the nitride film 18b. The blocking oxide film 18c blocks the migration of electrons to the gate electrode 18d while the electrons are trapped.
In this conventional semiconductor memory device, binary scale information can be stored and read using the characteristic that the threshold voltage varies depending on whether electrons are trapped in the trap site of the nitride film 18b. 
Here, when the density of a trap site increases, more electrons can be trapped, and the variation of the threshold voltage can be increased. That is, the density of the trap site can significantly affect the characteristics of the memory device. Conventionally, to increase the density of a trap site, techniques of scattering or depositing nano scale particles, such as Si nanoparticles, on the surface of a thin film have been developed. However, these methods can only provide limited increases in the density of the trap site per unit area. These methods have various technical problems, especially in the uniformity of particle size and particle distribution, to be applied to a flash memory.